Phosphorus-copper (phos-copper) and phosphorus-copper-silver (phos-copper-silver) alloys, identified by the American Welding Society (AWS) classification BCuP 1-9 brazing alloys (hereinafter “BCuP brazing alloys”), have been in wide use to join copper tubing and copper alloys together for over the past 70 years. Because these brazing alloys make strong ductile joints that resist vibration stresses, they are in wide use for the manufacture, installation and repair of air conditioning and refrigeration systems. Phosphorus and silver are used in the BCuP brazing alloys primarily to reduce brazing temperatures. The silver content typically ranges from 0% to 18% by weight, while the phosphorus content is typically 4-7.5%. Advantageously, phosphorus is limited to about 7.1 percent in the phos-copper alloys and about 6 percent in the phos-copper-silver alloys. Nickel, silicon and tin have also been added to the BCuP brazing alloys. When used to braze copper metal, the phosphorus in the BCuP brazing alloy is known to act as a fluxing agent, removing copper oxides sufficiently to effect wetting and brazing of copper. Because the phosphorus deoxidizes the copper surfaces, the BCuP brazing alloys are self-fluxing when brazing copper parts. Brasses and other copper alloys, however, require the use of adequate brazing fluxes. BCuP 1-9 brazing filler metals are not suitable to braze iron, cast iron, steels or other ferrous metals.
Carbon and low alloy steel, cast iron, stainless steel and nickel metals are generally brazed with AWS classification BAg 1-39 silver brazing filler metals. The BAg brazing alloys comprise silver, copper, zinc and may also include nickel and/or tin. The silver content ranges from 24-93% by weight, with 35-56% being most common. Zinc content ranges from 4-35% by weight, with 13-30% being most common. Torch brazing of ferrous metals requires the use of a brazing flux in the brazing process. Brazing fluxes remove oxides from ferrous metal surfaces to facilitate wetting of the ferrous metals, but become inactive at temperatures that exceed 1700° F. (927° C.). BAg-7 with 56% Ag, 17% Zn, 5% Sn and 22% Cu is exemplary, with a liquidus temperature of 1205° F. (652° C.) and a brazing temperature range of 1205-1400° F. (652-760° C.). Without consideration for cadmium bearing, toxic BAg 1, 1a, 2, 2a, and the like, silver brazing filler metals, BAg 7 is believed to be the most highly used silver brazing filler metal in the western world. It is not favored by its costly 56% content of silver, but by its lowest liquidus temperature of 1205° F. (652° C.). BAg-7 has the largest range of temperature between the start of becoming molten, and the higher temperature that causes the brazing flux to burn sufficiently to halt the flow of the molten silver brazing filler metal. Increasing silver content over 56%, in silver brazing filler metals, causes the liquidus to rise above the temperature of BAg-7.
The BCuP brazing alloys have only been considered suitable for brazing copper or copper alloys to copper or copper alloys. On the other hand, the BAg brazing alloys have been considered suitable for brazing a wide variety of base metals, to both like base metals and different base metals. For example, BAg brazing alloys can be used to braze a cast iron base metal to another base metal of cast iron, copper, copper alloys, nickel, nickel alloys, stainless steel, or carbon or low alloy steels.
There is a small, but general consensus in the brazing industry that the reason the BCuP brazing alloys will not braze steels is that phosphorus, contained in the braze alloy, causes the braze area to become brittle.
The present inventor does not concur with that opinion and finds that the entire range or group of BCuP brazing alloys do braze ferrous and non ferrous metals together. The confusion arises from the fact that the viewer of a live braze sees the molten braze alloy lose some of its surface tension and flow, by apparent capillary action, into the capillary area to fill and complete the braze. But when tested, the brazed parts are separated with an audible snap, and deemed brittle. However, phosphorus, when added to copper and silver in amounts up to 7.5%, the highest percentage contained in the BCuP brazing alloy group, reduces ductility, but not sufficiently to cause the brazed joint area to become brittle.
Generally, phosphorus is limited to about 7.5% in both the non silver bearing BCuP brazing alloys and the silver bearing BCuP brazing alloys. The BCuP brazing alloys, as cast and worked into finished new unused brazing wires are ductile and are not considered to be brittle. For example, testing of annealed 7.1% phos copper wire, BCuP 2, worked to a 36% reduction in size, resulted in a 75% hardness condition. A BCuP 5 wire of 5% phosphorus, 15% silver and 80% copper, annealed and worked to a 42% reduction in size caused the finished wire to become half hard. Both wires were springy and not overworked. After brazing steels and dissimilar metals with BCuP brazing alloys, the brazing alloy area of the brazed joint is fully annealed by the heat of the brazing process and is ductile. It is therefore believed that the failure of the brazed joint is due to a reason other than brittleness.
There is thus a need to identify the real cause of braze failure of the BCuP brazing alloys or other similar alloys when brazing metals other than copper, and to then enable an expansion of the use of BCuP brazing alloys and other similar alloys to other brazing applications.